Apple just posted the iMac Update 1.1, which makes improvements in the USB drivers. According to the read me:

This update improves the iMac’s ability to identify USB devices when starting up, improves the startup time when many USB devices are connected, and enables new USB solutions. You don’t have to install iMac Update 1.0 before you install iMac Update 1.1.

The updater will not run unless you have Mac OS 8.5 installed and the iMac Firmware Update installed, which Apple also just posted. According to the read me:

The iMac Firmware Update shold be installed on all iMacs to prevent problems with starting up your computer